Having written and checked hundreds of megabytes across four DOMs and two machines, after formatting successfully with HForm, I'm fairly satisfied it's actually reliable in practice with normal ADFS use. It's only initialising with PM that seems to be a problem. Patched ADFS made no difference, but I also suspect the issue may have been RISC OS 3.70 as things 'seemed better' with 4.39. Too much time and aggro spent to try and dig deeply in to finding the specific combination of things that upset things just now. Also, once a DOM had been successfully formatted with HForm, PM's error messages changed - that's why I ended up using 4 'factory fresh' DOMs.
I do agree that 'ADFS is the cause of the problem' but, as observed, it was impossible to get going using PM, whereas HForm has let me use the DOM, for better or worse.